The structures of most of the commercial supporting apparatuses, which can be folded in compact bundle, are articulated with poles, hinge and supporting textile apparatuses at times cooperating, such as the popular crossed style chairs, whose infinite variations are available FIG. 1. Their weight is on average from 2.5 to 4 kilograms and derives from thousand-year old structural models.
A version of three-legged camping stool FIG. 2 is composed of three poles joined on the upper part on the extremes to a piece of fabric, a hinge on three axes, in the middle of the poles ending on the ground without other constraints. Essentially this structural diagram copies the same structure of the seat of FIG. 1 wherein most of the elements of the frame are heavily strained by bending (1), shearing and torsional stresses. The consequence is a high materials expense compared to the unit of sustained weight. So these models are very heavy or very fragile or result quite low.
Furthermore the big dimensions of the materials involve high weight and significant volumetric encumbrances, therefore these objects neither are “pocket size” nor are “portable” meant as the possibility of storing them in a small rucksack or in a small bag. Many of these apparatuses are at most “transportable” that means they can be folded and stored in a car boot or they can be carried over one's shoulder for a short way. An example of the limits of these apparatuses is given by a tiny stool FIG. 3 that is only 8 or 9-inch high and has a coplanar closure of joined elements of traction (2) which are adjustable by a hook with the possibility of pre-tightening. Most of the elements of the structure are composed of full iron and compression exposes them to different strains. This microchair weighs a little less than one kilogram anyway, and has a limited functionality.
An American patent made by A. F. Draper in 1938 (U.S. Pat. No. 2,139,673) FIG. 4 probably didn't commercially succeed, but significantly improves the structural economy of foldable objects: a metal structure, in any case heavy because of the technological limits of the time. A polyhedral shell of means of traction composed of different materials contains the structure. Eight poles converging to a central junction hinge in pair and to every pole going up, corresponds another one going down.
Draper's seat structure not only has a doubtful effectiveness and complicated mechanics, but also has a significant problem: the shell of the means of traction strains all parts of the apparatus in the right way, but the four side tensioners are put under stress, in case of torsion between the supporting surface (3) and the one on the ground (4), only when the supporting surface has begun to rotate and the upper and lower extremes of the side means of traction move away one another. This effect makes the seat unstable and damages the hinges, because the stresses they are subjected to are not orthogonal to the angles of their pins, so the torsion damages the poles.
Regarding commercial foldable camping beds, the ratio of volume to maximum loading is even more significant: the average weight of these apparatuses, in order to hold a person who weighs 80 kilograms, is about 5 kilograms. In case of folding or detachable hammock stands, it raises at least to 12 kilograms. These weights discourage people carrying these apparatuses without using a means of transport. It is still a question of structures composed of elements that are exposed to great leverages and high torsional stress.